Semiconductor emitting devices, such as light emitting diodes (LEDs) and laser diodes (LDs), include solid state emitting devices composed of group III-V semiconductors. A subset of group III-V semiconductors includes group III nitride alloys, which can include binary, ternary and quaternary alloys of indium (In), aluminum (Al), gallium (Ga), and nitrogen (N). Illustrative group III nitride based LEDs and LDs can be of the form InyAlxGa1-x-yN, where x and y indicate the molar fraction of a given element, 0≦x, y≦1, and 0≦x+y≦1. Other illustrative group III nitride based LEDs and LDs are based on boron (B) nitride (BN) and can be of the form GazInyAlxB1-x-y-zN, where 0≦x, y, z≦1, and 0≦x+y+z≦1.
An LED is typically composed of layers. Each layer has a particular combination of molar fractions (e.g., given values of x, y, and/or z) for the various elements. An interface between two layers is defined as a semiconductor heterojunction. At an interface, the combination of molar fractions is assumed to change by a discrete amount. A layer in which the combination of molar fractions changes continuously is said to be graded.
Changes in molar fractions of semiconductor alloys allow for band gap control and are used to form barrier and quantum well (QW) layers. A quantum well comprises a semiconducting layer located between two other semiconducting layers, each of which has a larger band gap than the band gap of the quantum well. A difference between a conduction band energy level of a quantum well and a conduction band energy level of the neighboring semiconductor layers is referred to as a depth of a quantum well. In general, the depth of a quantum well can differ for each side of the quantum well. A barrier comprises a semiconductor layer located between two other semiconductor layers, each of which has a smaller band gap than the band gap of the barrier. A difference between a conduction band energy level of a barrier and a conduction band energy level of a neighboring semiconductor layer is referred to as barrier height. In general, the barrier height of a barrier can differ for each side of the barrier.
A stack of semiconductor layers can include several n-type doped layers and one or more p-type doped layers. An active region of an LED is formed in proximity of a p-n junction where electron and hole carriers recombine and emit light. The active region typically includes quantum wells and barriers for carrier localization and improved radiative recombination. Inside a quantum well, electrons and holes are described quantum mechanically in terms of wave functions. Each wave function is associated with a local energy level inside a given quantum well. An overlap of electron and hole wave functions leads to radiative recombination and light production.
A group III nitride LED is typically grown as a wurtzite or zinc blende crystal structure. At a heterojunction, the lattice mismatch of the two semiconductor layers causes stresses and strains of the crystal layers and leads to the development of a built-in electric field. In addition, a wurtzite crystal structure exhibits internal electric fields due to spontaneous polarization. The internal electric fields can lead to reduced overlap of electron and hole wave functions and, as a consequence, to reduced light emission.
Furthermore, the stack of semiconductor layers are typically grown on a sapphire or silicon carbide substrate structure. A large lattice mismatch between the substrate and the semiconductor layers can cause dislocations, which reduce the light emission of the device.